High temperature melt integrity microporous separator membranes for lithium ion batteries exist in the art. Japan Patent Application No. JP 2014141638 discloses an aromatic-polyamide (aramid) porous membrane which claims to provide heat resistance in a non-aqueous secondary lithium ion battery. The aramid polymeric binder proposed as a high temperature coating requires a non-aqueous solvent such as N-methyl pyrrolidone (NMP) in order to form a coating solution. An aramid is not soluble in water. The coating process involves a phase separation in order to form a porous coating on a membrane. In addition, an aramid may be required to be mixed with a polymer containing a polar substituent due to the aramid's tendency to aggregate in solution and its difficulty to form a uniform coating on a porous membrane.
U.S. Patent Publication No. US2014/0141314 proposes a (meth)acrylic polymeric binder with the ceramic particles in an aqueous medium as the coating slurry for a polymeric ceramic coating for a microporous separator membrane for a lithium ion rechargeable battery. However, certain acrylate polymeric binders may be prone to chain scission along the backbone of the vinyl polymer in certain electrolytes in a lithium ion battery and therefore may not be highly chemically stable polymeric binders in certain applications. Furthermore, U.S. Patent Publication No. 2014/0141314 proposes that spherical shaped ceramic particles with fine surface irregularities may be preferred in order for a (meth)acrylic polymeric binder/ceramic coating to have adhesion between the ceramic particles and the binder in the coating layer and, in addition, to achieve adhesion of the polymer ceramic coating layer to the porous base membrane so that ceramic particles do not flake off when the membrane is handled.
U.S. Patent Publication No. US2011/0229768 discloses a method for modifying a microporous separator membrane for use in a polymeric ceramic coating for a microporous separator membrane for a lithium ion rechargeable battery. This method uses a polymeric binder that is prepared by the polymerization of a mixture of several hydrophilic and hydrophobic monomers in a variety of monomeric ratios and combinations. The colloid coating slurry requires a plasticizer, a polyethylene wax powder and a surface treated ceramic particle in order to prepare a stable emulsified coating slurry. The objective of applying the coating slurry onto a microporous separator membrane is to improve the dimensional integrity of a microporous separator membrane and to reduce the shrinkage of a microporous separator membrane at high temperature, which might allow contact between the anode and cathode resulting a short circuit in a battery. However, the coated examples in U.S. Patent Publication No. US2011/0229768 achieved only an 18-24% thermal shrinkage at 150° C. which may not be a sufficient level of reduction in the thermal shrinkage to provide the needed level of safety required at high temperatures in certain high energy rechargeable lithium ion batteries.
A ceramic coated composite separator membrane with low or zero thermal shrinkage may be highly desirable for use in Li-ion batteries, especially for electric vehicles and high energy and power applications. Safety is a top priority for secondary lithium ion batteries.
Therefore, for at least certain applications, a need exists for a novel or improved coating formulation that not only provides a clean, environmentally friendly coating process that uses water or a mixture of water as the solvent in the coating slurry, but also uses a polymeric binder that has high thermal stability for extended high temperature shutdown, reduced thermal shrinkage with improved and/or excellent dimensional integrity, and/or oxidation stability when used in a lithium battery, such as a high energy rechargeable lithium ion battery.